Fig. 2. Conditioned place preference (CPP) following a single exposure or three consecutive exposures to nicotine. Fish showed a concentration-dependent change in preference for the treatment side following both a single exposure (grey bars) and three repeat exposures to nicotine on each of three consecutive days (black bars). The CPP response to 6 µmol l^–1 nicotine after a single exposure was not determined. Following exposure to 0, 3, 6, 30 and 150 µmol l^–1 nicotine for 20 min on each of three separate days fish showed a significant increase in place preference for the treatment side compared with before treatment (^**P<0.05). Fish subject to three treatments with 6 or 30 µmol l^–1 nicotine showed a significantly greater change in place preference for the treatment side than control, water-treated fish (^*P<0.05). Three exposures to 300 µmol l^–1 nicotine induced a significant decrease in place preference compared with water-treated controls (^*P<0.05).